1. Field of the Invention
This invention is directed to methods for polishing implantable medical devices, such as stents, for lower thrombogenecity and improved mechanical performance.
2. Description of the State of the Art
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially compress against the atherosclerotic plaque of the lesion to remodel the lumen wall. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient's vasculature.
A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings which can collapse and occlude the conduit after the balloon is deflated. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may require another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, a stent is implanted in the lumen to maintain the vascular patency.
Stents are used not only as a mechanical intervention but also as a vehicle for providing biological therapy. As a mechanical intervention, stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of a passageway. Typically, stents are capable of being compressed or crimped, so that they can be inserted or delivered through small vessels via catheters, and then expanded or deployed to a larger diameter once they are at the desired location. In addition, biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. One proposed method for medicating stents involves the use of a polymeric carrier coated onto the surface of a stent. A blend which includes a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent. The solvent is allowed to evaporate, leaving on the stent surface a coating of the polymer and the therapeutic substance impregnated in the polymer.
Stents have been made of many materials including metals and polymeric materials such as plastic, including biodegradable plastic materials. Stents have been formed from wire, tube stock, etc. Stents have also been made from sheets of material which are rolled into a cylindrical shape. A medicated stent may be fabricated by coating the surface of either a metal or polymeric scaffolding or substrate with a polymeric carrier. A drug can also be incorporated into a polymer from which a stent is made. In addition, the structure of a stent is typically composed of a pattern that allows the stent to be radially expandable. The pattern should be designed to maintain the necessary longitudinal flexibility and radial rigidity of the stent. Longitudinal flexibility facilitates delivery of the stent and radial rigidity is needed to hold open a bodily lumen.
The biocompatibility of an implantable medical device, such as a stent, is extremely important for successful treatment of a bodily lumen. One measure of biocompatibility is the tendency for an implantable medical device to form thrombus. The surface finish of an implantable medical device is an important factor in thrombus formation. Certain surface features such as cracks, pits, or jagged edges substantially increase formation of thrombus. Such imperfections tend to be a by-product of a fabrication process. In addition, imperfections in the surface of an implantable medical device may cause mechanical instability. Surface cracks or other imperfections tend to serve as sites at which stress applied to an implantable medical device is concentrated. Therefore, imperfections can result in the enlargement of existing cracks or formation of new cracks. This can occur when stress is applied to the implantable medical device, for example, during crimping or deployment.